1. Urinary excretion of dopamine (DA) increases during dietary salt loading. The majority of urinary DA is derived from circulating dihydroxyphenylalanine (dopa). Whether the increase in urinary DA excretion during salt loading results from increased efficiency of uptake of dopa by proximal tubular cells of the kidney, facilitation of intracellular conversion of dopa to DA, or increased delivery of dopa to tubular uptake sites, has been unknown.
2. In 10 inpatient normal volunteers on a constant diet, daily excretion of dopa and DA was assessed during normal sodium intake (109 mmol/day) for 1 week, low sodium intake (9 mmol/day) for 1 week and high sodium intake (249 mmol/day) for 1 week.
3. Urinary DA excretion exceeded urinary dopa excretion by about tenfold, and the excretion of both DA and dopa increased by about twofold between the low and high salt diets, with similar proportionate changes. Plasma dopa was unchanged by dietary salt manipulation.
4. The results indicate that increases in urinary DA excretion during dietary salt loading can be accounted for by increased delivery of dopa to sites of uptake by proximal tubular cells. Since dopa is released into the bloodstream by sympathetic nerve endings and by the brain, and since interference with decarboxylation of dopa attenuates natriuretic responses, dopa may function indirectly as a neurohormone involved in homoeostatic regulation of sodium balance.